Electrochemical measuring cells are widely used in the analysis of substances, where potentiometry, voltammetry/polarography, coulometry and conductometry can be mentioned as the most important principles of measurement. The use of electrochemical measuring cells for the analysis of gases has also been known for a long time. However, efforts are made to develop new, more sensitive and more reliable sensors, especially when toxic gases in the parts per billion (ppb) range are to be detected and optionally even quantitatively determined.
Such electrochemical gas sensors are required to meet the following requirements for highly sensitive analysis:                Low residual current I0;        no or at most only very weak effect of variations on the humidity and/or temperature of the air on the residual current I0;        low cross sensitivity to interfering gases;        low double layer capacity of the measuring electrode, especially in relation to dynamic measurement methods; and        high long-term stability.        
The properties of an electrochemical gas sensor are decisively determined by the material, the morphology and the layer thickness of the measuring electrode.
Platinum, gold or graphite are mentioned as measuring electrode materials, e.g., in U.S. Pat. No. 3,795,589. Very many gases can be reacted directly, i.e., without a mediator, on the catalytically highly active precious metals platinum and gold. Therefore, it is frequently impossible to reach the desired selectivity. The long-term stability of the catalytically less active graphite electrodes is slow and these show, depending on the electrode potential, high cross sensitivity to NO and NO2.
DE 199 39 011 C1 discloses a sensor, whose measuring electrode consists of diamond-like carbon (DLC). DE 101 44 862 A1 pertains to a measuring electrode made of boron-doped diamond (BDD). These electrode materials possess the properties required above, but they require a mediator, which selectively reacts with the analyte.
The use of ferroin (1,10-phenanthroline iron(II) sulfate) as a mediator is proposed in U.S. Pat. No. 3,795,589. However, to make reaction with SO2 possible, ferroin must be oxidized in the sensor to oxidation state III. This means long running-in periods and high residual currents (“cross-talk” with the auxiliary electrode), which are to be avoided.